1. Field of the Invention
The present invention relates to a Group III nitride semiconductor optoelectronic structure and a fabrication method thereof, and relates more particularly to a semiconductor optoelectronic structure with increased light extraction efficiency and a fabrication method thereof.
2. Description of the Related Art
Light generated by the active layer in a traditional light emitting diode structure cannot be completely emitted outside the light emitting diode structure because light may partially transmit in a lateral waveguide and be totally reflected in the light emitting diode structure. In addition, internal constituents such as an active layer, a buffer layer, defects of material, and metal electrodes may absorb a portion of the light. Thus, the light extraction efficiency of the traditional light emitting structure is low.
Using a GaN-based Group III nitride blue-and-white-emitting light emitting diode as an example, the critical angle at which total reflection occurs for a uniform interface between the GaN (gallium nitride) material with a refractive index of 2.5 and the air with a refractive index of 1 is calculated to be 23.5 degrees. Therefore, light emitted from the active layer of a GaN light emitting diode and incident on the interface at an angle greater than 23.5 degrees is completely reflected back to the active layer, and reflected within the active layer until the light is completely absorbed.
One method to reduce the amount of light undergoing total reflection within the active layer is to generate irregular or roughened structures on the active layer for scattering light. However, because the p-type GaN layer on the active layer is very thin, the dry etching depth and the impairment caused by plasma ions cannot be effectively controlled during the etching process. Other methods are to roughen the surface of a light emitting diode or the side surface of a cutting area for increasing the light extraction efficiency.
FIGS. 1A and 1B show a roughened structure on a surface of a light emitting diode disclosed in U.S. Pat No. 6,441,403. Referring to FIG. 1A, the method provided by U.S. Pat. No. 6,441,403 forms an epitaxial layer 117 on a sapphire substrate 101. When a p-GaN layer 109 is formed, the temperature is lowered, the growth speed is adjusted, and the ratio of Group III element to Group V element is adjusted so that a rough surface is obtained. Next, an etch process is performed to expose the n-GaN layer 105 and to form a cutting area. An n-type electrode 113 and a p-type electrode 115 are separately formed on the p-GaN layer 109 and the n-GaN layer 105. Finally, a plurality of chips is obtained through a cutting process. Further, in the epitaxial layer 117 of FIG. 1B, the n-GaN layer 105 can be formed after the formation of the p-GaN layer 109. On the n-GaN layer 105, an n-GaN layer 111 having a rough surface can be formed. In the method, a single surface is roughened; although the light extraction efficiency can be improved, some light may still enter into the sapphire substrate 101 and be confined within the light emitting diode. Moreover, an electrode directly formed on a rough surface may cause a device driving voltage increase issue.
FIGS. 2A and 2B show a light emitting diode structure disclosed in U.S. Pat. No. 7,053,420. Referring to FIG. 2A, a concave and convex surface 203a is initially formed on a sapphire substrate 201. A buffer layer 205 with different refractive index can next be formed on the concave and convex surface 203a. A semiconductor layer 213, including an n-type conduction layer 207, a light-emitting layer 209, and a p-type conduction layer 211, is then formed on the buffer layer 205. Further, the difference between the structure of FIG. 2A and the structure of FIG. 2B is that the concave and convex surface 203b in the structure of FIG. 2B has a triangular wave shape. The patent teaches that one surface of the sapphire substrate is processed to form a concave and convex surface. Because the sapphire substrate has characteristics such as surface hardness, high thermal stability, and stable chemical properties, the processing of the sapphire substrate is difficult. In addition, nitride semiconductor has refractive index of 2.3, and a sapphire substrate has refractive index has refractive index of 1.8. The two differ in a value of 0.5.Consequently, a portion of light may enter into the sapphire substrate without being efficiently utilized.
FIGS. 3A to 3C and FIGS. 3A′ to 3C′ demonstrate a process method disclosed in a paper entitled “Improved luminance intensity of InGaN-GaN light-emitting diode by roughening both the p-GaN surface and the undoped-GaN surface,” APPLIED PHYSICS LETTERS 89, 041116 (2006). The process method combines a surface-roughening technique, a wafer-bonding technique, and a laser lift-off technique to manufacture a light-emitting diode having two rough surfaces. Referring to FIG. 3C, the process method forms an epitaxial layer 311 on a sapphire substrate 301. Next, using inductively coupled plasma process, the method dry etches a p-GaN layer 307 to obtain a rough surface. Next, a transparent conduction layer 309 is formed on the rough surface of the p-GaN layer 307. Next, a p-type electrode 313 is formed on the transparent conduction layer 309, and an n-type electrode 315 is formed on an undoped GaN layer 305. Thereafter, a laser is applied to separate the sapphire substrate 301 from the epitaxial layer 311. The undoped GaN layer 305 is then wet etched, and an adhesive layer 303 is applied for bonding the epitaxial layer 311 and the sapphire substrate 301 together so that two rough surfaces are obtained. FIG. 3A shows a general light-emitting diode having no processed light output surface. FIG. 3B shows a light-emitting diode having a single rough surface. The light output surface perpendicular to the light-emitting direction is roughened. FIG. 3C shows a light-emitting diode having two rough surfaces. The light output surface of the p-GaN layer perpendicular to the light-emitting direction and the surface of the undoped GaN layer opposite to the light output surface are roughened.
Comparing the light extraction efficiency of the above three structures, FIG. 3A′, corresponding to FIG. 3A, shows the behavior of photons in a general light-emitting diode. Due to the planar light output surface, photons incident at an angle greater than 23.5 degrees will be reflected back to the active layer so that the light extraction efficiency is low. FIG. 3B′, corresponding to FIG. 3B, shows the behavior of photons in a light-emitting diode having a single rough surface. It can be seen that the rough surface does not limit the emission of photons to an angle below the critical angle of 23.5 degrees. Therefore, the light emitting efficiency can be improved. FIG. 3C′, corresponding to FIG. 3C, shows the behavior of photons in a light-emitting diode having two rough surfaces. Roughening the surface of the undoped GaN layer can reflect photons so as to further improve the luminance intensity of the rough light output surface, to further improve the light extraction efficiency compared to that of the structure of FIG. 3B. Although the above method can improve the light extraction efficiency, the light-emitting diode needs two bonding steps, resulting in process stability and manufacturing yield issues.
Thus, the method of the present invention does not have the above-mentioned drawbacks, and can improve the light extraction efficiency of a light emitting diode. Further, the method of the present invention can reduce the intensity of defects in an epitaxial layer, thereby improving the quality of the epitaxial layer.